It must be accepted that, in normal use, components made of insulating material may become moist and dirty. If the impurities are electrically conducting, current will flow be-tween parts of different voltage potentials - the tracking current. As a result, small flash-overs may occur, which cause thermal stresses to the surface of the insulating material.

The visual result of thermal decomposition of the insulating material caused by the tracking current is called the track. The interconnecting formation of carbonized material forms a tracking path, by which a lasting current conducting connection results. Thereafter the component cannot fulfill its intended electrical function in the majority of cases and parts may catch fire. The resistance of an insulating material to track formation is called the tracking resistance.

Test Method

The behaviour of the surface of solid insulating materials under the influence of tracking currents is to be determined as follows: The tracking currents are produced by allowing drops of an electrically conductive solution to fall centrally between two electrodes which are put on the sample and subjected to an alternating voltage.

The tests can answer the following questions:

· What highest voltage causes no breakdown up to 50 drops, while another voltage, 25 volts lower than the previously used voltage, causes no breakdown up to 100 drops? (Method CTI).

· Does no breakdown occur at a given voltage up to 50 drops each? (Method PTI).

Testing Apparatus

Platinum is specified as a material for the elec-trodes. The power supply unit is designed for voltages up to 600 V, resp. up to 1 000 V. An electronic control unit serves for releasing the drops.

In the case of the PTL tracking test apparatus, the electrode arrangement is mounted in a cabi-net with polycarbonate panes. By two wing doors, a good accessibility of the testing ar-rangement ensures a pleasant working. A safety system breaks the supply when the cabinet is opened.

1 timer for the electromagnetic valve for an impulse every 30 seconds, impulse width adjustable for setting the drop size, with resettable counter for switching off the apparatus, with digital display for the testing time in minutes and seconds,